When an audio tape becomes suspected of tampering, it may be forwarded to a qualified forensic audio specialist for authentication. Examples of such problems are: Credibility questions relating to the tape recorder operator, chain-of-custody contradictions, and differences between the content of the tape and testimonies of what was said.

Most often a forensic expert is contacted when the tape is believed to have been altered or tampered with. Due to the nature of the allegations surrounding tampering issues, the examiner required specific items from the patron. The Federal Bureau of Investigation, for example, has a list of required information including:

The original tape.
The tape recorders and related components used to produce the recording.
Written records of any damage or maintenance done to the recorders, accessories, and other submitted equipment.

A detailed statement from the person or persons who made the recording, describing exactly how it was produced and the conditions that existed at the time, such as:

Power source, such as portable generator or dry-cell batteries.
Input, such as telephone, radio frequency transmitter/receiver, miniature microphone, etc.
Environment, such as telephone transmission line, restaurant, apartment, etc.
Background noises, such as television, radio, unrelated conversations, computer games, etc.
Foreground information, such as number of individuals involved in the conversation, general topics of discussion, closeness to microphone, etc.
Magnetic tape, such as brand, format, when purchased, whether previously used.
Recorder operation, such as number of times turned on and off in the record mode, type of keyboard or remote operations for all known events, use of voice-activated features, etc.
A typed transcript of the entire recording or, if that is not available, transcriptions of the portions in question.

The items listed above are examples of what is required by a forensic expert as he begins an examination of questioned audio recordings.

Falsification of Tapes
A qualified forensic expert determines authentication by performing a number of scientific tests which detect evidence of tampering or falsification. Four basic types of tampering include:

Deletion - the elimination of words or sounds by stopping the tape and over-recording unwanted areas.

Obscuration - the mixing in of sound of amplitude sufficient to mask wave form patterns which originally would show stops and starts in inappropriate places.

Transformation - the rearranging of words to change content or context.

Synthesis - the adding of words or sounds by artificial means or impersonation.

Electromechanical Indications of Such Falsification (Anomalies) Would Include:
Gaps - segments in a recording which represent unexplained changes in content or context. A gap can contain buzzing, humming, or silence.

Transients - short, abrupt sounds exemplified by clicks, pops, etc. Transients may indicate tape splicing.

Fades - gradual loss of volume. Fades can cause inaudibility and are considered gaps when the recording becomes fully inaudible.

Equipment Sounds - inconsistencies of context caused by the recording equipment itself. Common equipment sounds include hums, static, whistles, and varying pitches.

Extraneous Voices - background voices which at times appear to be near as the primary voices. These can at times even block the primary voices.

Methods FTA Uses to Detect Falsifications and Authenticate Tape Recordings:
A forensic expert is trained to correlate his observations of such anomalies with machine functions to interpret events in the following ways:

Critical Listening: Use of human analytical capabilities to locate anomalies. The forensic scientist listens with proper headphones to the original tape using high quality analytical equipment. He first performs a preliminary overview of the original tape and notes recorded events including starts, stops, speed fluctuations, and other variations requiring further investigation. He then examines record events and categorizes them as environmental or non-environmental. After examining any anomalies, the expert analyzes background sounds. He listens for abnormal changes, absences, or differences in environmental sounds. The final phase of critical listening is an extensive audit of the foreground information. He concentrates on voices, conversation and other audible sounds. Here anomalies include sudden changes in a person's voice, abrupt unexplained topic change, or strong foreground interruptions indicative of obscuration. The initial forensic process of critical listening provides foundation and direction for later intensive tests.

Physical Inspection: The forensic expert next inspects for tampering with a thorough visual inspection of the tape itself. He inspects the housing for pry marks, welding, size, label and date consistent with alleged recording date. He also measures the tape and assures the splicing of magnetic tape to the leader is consistent with normal manufacturing process. Any other splices are noted as possible alteration.

Magnetic Development: Direct visual observation of the magnetically "developed" tape is conducted to find track widths, the type of recorder used, and the presence or absence of residual speech signals.

Spectrum Analysis: Specialized computer equipment and programs to produce a visual interpretation of a frequency-versus-amplitude and frequency-versus-amplitude-versus-time displays. This allows the expert to view the entire spectrum or to zoom in on an area of particular interest thereby helping to characterize the acoustic quality of anomalies and identify their source.

Waveform Analysis: A computer generated display representing time-versus-amplitude of recorded sounds in graphic form. With such analysis the expert can sometimes measure signal return time which reveals how long a recorder had been turned off. He can identify record-mode including the measurement of record-to-erase-head distances, determination of the spacing between gaps in multiple-gap erase heads, and inspection of the signature shape and spacing of various record event signals.

Recorder performance: various electrical and mechanical measurements of standard and modified recorders for use in finding possible origins of buzz sounds, hum, etc.

In order to submit sound recordings as evidence in court proceedings, an attorney must prove that the tape is an authentic representation of the conversation it is said to record. The traditional method of establishing authenticity involves maintaining a chain of custody which logs all persons, times and locations concerned in the creation of the tape. However, even if this procedure is strictly observed, there may still be a challenge to the tape's authenticity.

The recording may contain inconsistencies suggestive of tampering. In such cases, an attorney may consult a qualified forensic examiner to inspect the tape. The examiner would initially listen critically for signs such as gaps, transients, fades, equipment sounds or extraneous voices which indicate tampering. Then he would utilize other methods like physical inspection, magnetic development, spectrum analysis, and waveform analysis to discover anomalies. It is relatively easy to change the content of a recording by deleting words or sections; by obscuring meaning with over-recorded sounds; or by transforming context through rearrangement of selected phrases or by adding additional words through synthesis. Nevertheless, falsifications normally leave detectable magnetic and waveform acoustic signatures which can lead to forensic individualization of the evidential recorders and tapes.

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